Did a couple of sub 34 (33:47 & 33:27 two years apart) when older than 30 while training for tri's. Raced in track primarily 800M & 1500M at uni and on club (1:50's & 3:50-55's). Ran a bunch of 5K road races at 16:00+/-.
Never trained for 5K & above on the track only the short stuff. Learned pacing there.
Trained for the tri run using fartlek for speed on Tuesdays (30-40 minutes total) and Saturdays (45-55 minutes total). Varying distances and speeds but pushed a lot of speed in Zone 4 (like 60-62 second estimated 400 meters and sub 2:30 estimated 800 meters) for 25-33% of those workouts when training for Olympic. These were continuous runs. Did LSD of 6-8 miles for endurance on Sunday's after 20-25 mile bike for Oly training. Ran every day 4-6 miles in Zone 2 on M, W, R, & F. When running hard days, some of the bikes and swims were varied Zone 2 stuff into Zone 3 for probably 80-90% of the time and 10-20% in Z4. Body could not train in Zone 4 across all 3 legs and stay healthy week after week.
Ran first marathon (sub 2:42) at age 33 on 5 weeks of running only and no more than 50 miles in a week. But
that came after a solid tri training for Oly distance all summer using the above workouts.
Did first half IM training starting at age 40. Increased running mileage on the Sat & Sun runs only by 25-40%. The Tuesday & Saturday fartlek runs were slower due to age but the intensity and distance the same. During this same time ran master track meets for three consecutive Junes hovering around 2:05 for 800M and 4:10 1500M so still had some leg speed but raced thru the meet with focus on tri training. Did no intervals on the track, all fartlek on the road or dirt. Increased weekend bike too about the same percentages while weekday stuff said steady at 60-75 minute rides. Also started masters swimming 3x week from being on my own for a decade.
Picked up training for IM's four years later in mid-40's and lost all the track speed capabilities.
Chose fartlek because that's how you race. Chose to train on the roads (or trails) because of the varying terrain and that's the race conditions RD's create for the competitors. Chose fartlek because track intervals got boring after 15+ years of organized track. Most of the college 10K track teammates did their workouts on the road. Their tests always came in the weekend meets.
Can't provide objective workout times and distances to race times and distances because everything was subjective: distances, terrain, intensity, training cycle, times, and whatnot. Ran 15 or so sub 4:50 IM 70.3's (down to in the high teens) over a 15 year period.
Consider fartlek in your workout portfolio. Hope this helps.